If you are invited to stay overnight at a friend’s house, you should make it clear when and how you will arrive and exactly when you plan to de–part. If you leave your plans unclear, it can prevent the household from making its own arrangements.
It is not obligatory to take a gift, but if you will be eating several meals at the home, you want to take a gift and present it as soon as you arrive. If there are children in the family, then an age–appropriate toy or book would be nice. Suggestions include a bottle of brandy or whiskey (unless the family does not drink alcohol!), a vase, a set of small guest towels, vegetables from your garden, a basket of fruit, a flower arrangement, a small plant, a Christmas decoration (prior to the holiday), or several bottles of wine. If you are a photographer, you might take pictures of your host family and send one of the nice ones in a frame afterwards.
American kids like to have friends come to spend the night. Whether these events are called sleep–overs, slumber parties, or pajama parties, the kids invariably stay up late at night watching TV or videos, playing games and talking. When a child is invited to such a party, the parents might want to send along some party food (snacks, drinks) for everyone to enjoy. Actual gifts are un–necessary, unless the occasion is to celebrate a birthday, because some day you will probably reciprocate by having the same children sleep at your home.